Usually it refers to the E6300 and E6400 CPUs, as Quiksilver said, which only had 2MB of L2 Cache. All other Conroes have 4MB. (CPU-z reports my E4500 as a Conroe-2M, although I think the E4xxx series are technically supposed to be called Allendales.) What the difference between Conroe-2M and Allendale is, I couldn't tell you.